The first requirement for a bike is one that fits. My first bike about 8 or so years ago was from a consignment shop and cost $65. This was a great bargain because it was in fine condition. Sad to say it was way too big and as I began to do longer rides the poor fit became painfully obvious. So I would recommend doing whatever you can to determine the right size. After size, wider tires than the typical road bike would be better for gravel. Probably a width of 35 mm (say 1 1/2 inches). Then mounting points for the child seat. This is quite a bit of information to track down so don't be in a hurry.